Abstract
During the winter of 2006 we measured nifH gene abundances, dinitrogen (N2) fixation rates and carbon fixation rates in the eastern tropical and sub-tropical North Atlantic Ocean. The dominant diazotrophic phylotypes were filamentous cyanobacteria, which may include Trichodesmium and Katagnymene, with up to 106 L−1 nifH gene copies, unicellular group A cyanobacteria with up to 105 L−1 nifH gene copies and gamma A proteobacteria with up to 104 L−1 nifH gene copies. N2 fixation rates were low and ranged between 0.032–1.28 nmol N L−1 d−1 with a mean of 0.30±0.29 nmol N L−1 d−1 (1σ, n = 65). CO2-fixation rates, representing primary production, appeared to be nitrogen limited as suggested by low dissolved inorganic nitrogen to phosphate ratios (DIN:DIP) of about 2±3.2 in surface waters. Nevertheless, N2 fixation rates contributed only 0.55±0.87% (range 0.03–5.24%) of the N required for primary production. Boosted regression trees analysis (BRT) showed that the distribution of the gamma A proteobacteria and filamentous cyanobacteria nifH genes was mainly predicted by the distribution of Prochlorococcus, Synechococcus, picoeukaryotes and heterotrophic bacteria. In addition, BRT indicated that multiple a-biotic environmental variables including nutrients DIN, dissolved organic nitrogen (DON) and DIP, trace metals like dissolved aluminum (DAl), as a proxy of dust inputs, dissolved iron (DFe) and Fe-binding ligands as well as oxygen and temperature influenced N2 fixation rates and the distribution of the dominant diazotrophic phylotypes. Our results suggest that lower predicted oxygen concentrations and higher temperatures due to climate warming may increase N2 fixation rates. However, the balance between a decreased supply of DIP and DFe from deep waters as a result of more pronounced stratification and an enhanced supply of these nutrients with a predicted increase in deposition of Saharan dust may ultimately determine the consequences of climate warming for N2 fixation in the North Atlantic.
Highlights
Nitrogen is a key nutrient, limiting primary production throughout much of the world’s upper oceans [1]
In the western part of the North Atlantic Ocean Trichodesmium biomass and N2 fixation rates are reported to be high [4,10], whereas in the eastern part of the North Atlantic N2-fixing unicellular cyanobacteria are reported to be responsible for a significant part of the N2 fixation [3,11,12]
Study area Our study area was situated between 12–30uN and 20–30uW, with most of the N2- and CO2 fixation experiments performed in proximity of the Cape Verde islands
Summary
Nitrogen is a key nutrient, limiting primary production throughout much of the world’s upper oceans [1]. The environmental variables available for analysis included: i) oceanographic variables such as oxygen, salinity, the water-mass as identified by a temperature salinity plot, and the mixed layer depth [25], ii) as factor the dust event on 3&4 Feb 2006, iii) NO3-, PO43-, SiO44-, NH4+, DON and DOP, iv) biological variables including the photosynthetic efficiency, Fv/Fm, and the PSII cross-section (sPSII), flow cytometer counts of heterotrophic prokaryotes, picoeukaryotes, Prochlorococcus, Synechococcus and the nifH gene abundance of gamma A proteobacteria, unicellular group A and filamentous cyanobacteria, and the N2 fixation rates, and v) trace metals DAl and DFe, free and total concentrations of Fe-binding ligands and their conditional stability constant log K’.
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